Immunoglobulin G (IgG) from patients with heparin-induced thrombocytopenia binds to platelets in an environment that contains approximately equimolar concentrates of platelet factor 4 (PF4) and heparin. The binding requires the F(ab1)2 end of the IgG but not the F end. The specific IgG binding to platelets can be isolated by binding to the PF4 that is attached to heparin-sepharose.We have more recently elucidated a detail about how complexes of heparin, PF4, and IgG bind to the platelet surface. Binding of the termolecular complexes occurs via the heparin component. When heparin is in excess, free heparin competes with the complexes and displaces them from the platelet membrane. This explains why the molar ratio of heparin to PF4 is critical in determining the in vitro and in vivo reactivity of the complexes with platelets.